Magma Chamber Recharge Research Articles

From magma replenishment to epithermal mineralization: Combined evidence from zircon U-Pb, (U-Th)/He and trace element data

The Dahalajunshan Formation volcano-volcanoclastic rocks in Chinese Western Tianshan host specific Paleozoic epithermal gold deposits as well as volcanogenic Fe-Cu mineralization. Although numerous studies have been taken to constraint its geological, geochemical, geochronological features as well as petrogenesis, the link between volcanism and mineralization is not well constrained. In this contribution, accessory zircon was collected from the intermediate to felsic volcanic rocks outcropped at the Jingxi-Yelmand gold deposit. Detailed cathodoluminescence imaging illustrates ubiquitous resorption surfaces and embayments, and in situ LA-ICPMS trace element analysis further indicates periodic recharge of magma chamber by mafic magma input from depths. U-Pb geochronology constrains two main pulses of magmatism at ~370 Ma and 365–362 Ma, respectively. The duration of magmatic activity in the magma chamber is estimated to 52 ± 6 Myrs, using the U-Pb age of the oldest zircon antecryst (406.9 ± 5.4 Ma) and the youngest zircon autocryst (354.6 ± 3.0 Ma) as cutting edge and taking the analytical uncertainty into consideration. (U-Th)/He dating of zircon from the first pulse yields an age of 354 ± 15 Ma, which is identical to U-Pb age within analytical uncertainty. However, zircons from the second pulse of volcanic rocks yield much younger (U-Th)/He centered at 292.6 ± 7.6 Ma due to significant post-eruption reheating events. We propose that the intermediate to acid volcanic rocks are constructed by incremental addition of small magma batches. The periodic injection of more mafic magma and subsequent mixing may act as important trigger of not only volcanic eruption but also epithermal mineralization. Gold mineralization at the deposit should be no later than 292.6 Ma.

Magmatic evolution of ediacaran alkali rhyolites from the Acampamento Velho volcanism in the Tupanci area, southern Brazil: A study based on mineral chemistry, LA-ICP-MS Ti-in-quartz and zircon saturation geothermometry

Post-eruptive, metamorphic, and weathering processes are issues that affect not only the identification of geochemical signature but also mineral paragenesis on ancient volcanic successions. Still, the estimate of pressure, temperature and oxygen fugacity conditions prevalent during magma crystallization can be achieved for volcanic systems of any age by combining experimental, thermodynamic and natural data using geothermobarometers. In this regard, highly resistant minerals such as zircon and quartz can provide key information on the origin and evolution of silicic magmatic chambers. For example, the substitution of Ti4+ for Si4+ in quartz is a function of the activity of TiO2 in the magma and in the crystal, temperature and pressure. Therefore, the aim of this study is to apply Ti-in-quartz (TitaniQ) coupled to Zircon saturation geothermometry to provide a better insight into the igneous processes that originated post-collisional Ediacaran sodic-alkaline rhyolites from the Acampamento Velho in southern Brazil. Zircon saturation geothermometry estimates range from ca. 825 °C–1000 °C (within an error of ~25 °C), in agreement with previously reported zircon crystallization temperatures for the Acampamento Velho volcanism in the Ramada and Taquarembó plateaus. Temperature estimates using the Ti in quartz thermometer range from 700 to 710 °C for the intrusive rocks and from 710 to 810 °C for the lava flows that show quartz zonation. The Ti zonation pattern in quartz was probably generated by partial phenocryst dissolution in response to a magma chamber recharge and increase in the pre-eruption temperature. This type of process could be similar to the injection of mafic melts rich in volatiles that originated giant magma bodies, indicating that the Acampamento Velho volcanism was likely formed in a supereruption event with associated caldera collapse, as previously evidenced by detailed petrography studies in other occurrences in the Sul-Rio-Grandense shield.

Petrography, relationships, and petrogenesis of the gabbroic lithologies in Northwest Africa 773 clan members Northwest Africa 773, 2727, 3160, 3170, 7007, and 10656

AbstractThe Northwest Africa (NWA) 773 clan of lunar meteorite stones are coarse‐grained breccias that provide an opportunity to examine a lunar igneous system that includes inferred intrusive and extrusive lithologies, possibly related through a common liquid line of descent from a single source region. Such extensive sampling of a single very low‐Ti (VLT) magmatic system on the Moon is unprecedented among the lunar samples. This study focuses on the olivine gabbro (OG), anorthositic gabbro (AG), and ferroan gabbro (FG) lithologies variably contained in NWA 773, NWA 2727, NWA 3160, NWA 3170, NWA 7007, and NWA 10656. Mineral compositions in the three gabbros indicate the crystallization sequence OG → AG → FG. Petrologic modeling of these three lithologies, and an olivine phyric basalt that also occurs in the NWA 773 clan, however, suggests that the relationship among the lithologies is more complex. The OG and basalt can be modeled as originating from a VLT KREEP‐bearing parental melt similar to the Apollo 14 Green Glass b1 composition through mainly equilibrium crystallization. The AG and FG, however, do not fit this simple model and require either a more complex crystallization sequence involving fractional crystallization, magma chamber recharge, or perhaps heterogeneity in the source region.

The Mount Cameroon southwest flank eruptions: Geochemical constraints on the subsurface magma plumbing system

With seven major eruptions in the 20th century, Mount Cameroon is one of the most active volcanoes in Africa. However, information on the volcano's plumbing system is still relatively scarce. In order to contribute to filling this knowledge gap, a geochemical, clinopyroxene geobarometry, and SrNd isotope study was carried out on samples collected from the 1922 and lower 1999 lava flows found on the SW flank of the volcano. Petrographic evidence shows a common crystallising mineral assemblage of olivine and clinopyroxene and/or plagioclase scattered in a microlite-rich groundmass. The total alkali-silica classification diagram shows that the lavas of 1922 exhibit basanitic compositions those of 1999 are predominantly basanites with some few hawaiites. The Sr isotope ratios for the 1999 samples in this study show values somewhat lower and higher than the limits obtained for historic Mount Cameroon samples so far. While major and trace element trends indicate fractional crystallisation processes, chemical and mineral disequilibrium textures are indicative of open-system processes such as magma mixing and magma chamber recharge. Variations in incompatible trace element ratios, which hardly change during fractional crystallisation, are also suggestive of open-system processes. Plots of K2O and Nb show a mixing trend between samples from these eruptions, implying a common magma chamber where remnant material from preceding eruptions was incorporated into succeeding eruptions. Clinopyroxene geobarometric calculations show this common dominant crystallisation depth at 23–29 km.

Improving deformation models by discounting transient signals in geodetic data: 1. Concept and synthetic examples

AbstractGPS geodesy provides very precise velocities of benchmarks on decadal timescales, and geodesists often describe their uncertainties with a velocity covariance matrix. However, those who model neotectonic deformation to estimate long‐term seismic hazard want constraints on the interseismic velocities of stable bedrock on multithousand‐year timescales. When the former (available data) are used as proxies for the latter (desired constraints), it is necessary to increase uncertainties to characterize a variety of transient and/or surficial noise processes, including magma chamber recharge, postseismic relaxation, pore fluid motion, extremely slow landsliding, and glacial isostatic adjustment. The effects of transient noise on distant reference benchmarks also add to uncertainty of the long‐term velocity reference frame. We augment the reported velocity covariance matrix with transpose products of velocity perturbation vectors from simple models approximately describing anticipated transient and surficial noise sources. No artifacts are introduced by this method because the velocity‐vector data are unchanged. When the inverse of the augmented covariance matrix (the “diminished normal matrix”) is used in the objective function of a neotectonic deformation model partially driven by GPS data, the perturbing effects of transient and surficial signals are greatly reduced. Improvement occurs even when the prior estimates of noise processes are rather crude. We present two examples computed with synthetic data.

Deep to shallow crustal differentiation of within-plate alkaline magmatism at Mt. Bambouto volcano, Cameroon Line

At Mt. Bambouto, a continental stratovolcano of the Cameroon Line, magmatic activity lasted for over 20Ma and was characterized by at least two caldera formation events. Here we present detailed mineral and whole-rock compositions of Mt. Bambouto basanites, hawaiites, trachytes and phonolites, with emphasis on caldera related volcanic rocks. These data show that differentiation took place within a complex magma plumbing system, with magma chambers occurring at different depths within the crust. Though differentiation was chiefly dominated by fractional crystallization, chemical mineral zoning of olivines, clinopyroxenes, and feldspars is also indicative of open-system processes such as magma mixing and magma chamber recharge. Chemical zoning is evident mainly in the outer 100 microns of the analyzed crystals, suggesting that magma mixing occurred shortly before eruption.The last caldera collapse at about 15Ma also marked a clear change in the magma plumbing system. Before caldera collapse, Mt. Bambouto was characterized by a dominant production of peralkaline quartz trachytic magmas in shallow magma chambers. During this phase, evolved basic magmas (hawaiites) and strongly evolved alkaline magmas were formed in middle and upper crustal magma chambers, respectively. After emptying of the shallow quartz trachytic magma chamber and caldera collapse, magmas from the deep magmatic plumbing system were mobilized and partially mixed. This triggered eruptions of magmas on the caldera rims.

Stratigraphy, thickness, tectonic environment, and economic implications of the giant Suwar – Wadi Qutabah layered mafic complex in northwestern Yemen

The Wadi Qutabah layered mafic intrusion in northwestern Yemen, considered part of a giant (>250 km2), Neoproterozoic (∼638.5 Ma) intrusion referred to as the Suwar – Wadi Qutabah Complex, has significant potential for economic platinum-group element and Ni–Cu–Co mineralization. A search for platinum-group element mineralization at Wadi Qutabah yielded cores from 14 drillholes along two east and west lines. Stratigraphically, they reveal ∼500 m of interlayered norites with subordinate gabbros, anorthosites, pyroxenites, and massive sulphide layers showing modal and cryptic layering that can be correlated between individual holes and between the east and west areas. Energy dispersive spectrometry major element analyses on mineral phases show gradual upward changes in plagioclase composition from An52 (lowest level drilled) to An44, and En61 to En57 in orthopyroxene, up to a distinctive, ilmenite-rich unit termed 5a (augite norite). Above 5a, minerals show a dramatic shift to more primitive An57 and En69 compositions. The gradual changes are typical of layered mafic intrusions and reflect progressive crystallization of magma upward. The dramatic changes, such as above 5a, are also common and reflect magma chamber recharge. In addition to the 500 m reported here, mineral exploration data from more-mafic rocks at Suwar suggest 400 m of layered rocks below those at Wadi Qutabah (total 900 m). A regression analysis plot of the cumulus mineral (plagioclase and orthopyroxene) compositional change versus thickness for seven well-known, giant, layered mafic intrusions suggests that the Suwar – Wadi Qutabah Complex is >2 km thick and one of the largest (thickness and area) intrusions on Earth. Comparison of the stratigraphy and mineral compositions at Wadi Qutabah with intrusions bearing PGE deposits suggests prospective areas for mineral exploration to the north and south of the study area. Recent reviews identify many similar-aged, mafic–ultramafic intrusive complexes across the Arabian–Nubian Shield. The large size of the Suwar – Wadi Qutabah Complex, its noritic composition, and subcontinental lithospheric mantle signature are characteristics typical of mafic rocks formed in extension-related, continental rift settings. Thus, the Suwar – Wadi Qutabah Complex may be part of a previously unrecognized large igneous province associated with a late Proterozoic magmatic event and preserved in what is today, the Arabian Shield.

Deformed Chromitite Layers in the Coobina Intrusion, Pilbara Craton, Western Australia

The Archean Coobina intrusion contains a number of tectonically dislocated and folded chromitite seams within a strongly deformed ultramafic-mafic layered intrusion. The intrusion consists of predominantly serpentinized olivine cumulates, overlain by gabbroic rocks. The entire sequence, along with the overlying greenstone belt sediments and volcanic rocks, shows evidence for multiple episodes of ductile deformation and a later phase of shearing associated with the Capricorn orogen. This history has resulted in isoclinal folding and boudinage of the chromite layers and disruption of original lateral continuity. This deformation appears to have been accommodated by micrometer-scale microshears within the massive chromitites rather than by penetrative fabric development. The Coobina chromitites have compositions at the more Cr-rich end of the range observed in layered intrusions worldwide. Chrome contents and Cr/Al ratios are indicative of a parent magma of komatiitic affinity. Mg/Fe ratios vary unsystematically and have probably been modified by varying degrees of subsolidus exchange of Mg and Fe with surrounding olivine-rich cumulates. Chromitites are consistently Pt and Pd poor, have typical 100-ppb levels of Ru and Ir, and show no systematic relationship to stratigraphy or major element chromite chemistry. The lack of Pt and Pd enrichment is attributed to formation from uniformly S-undersaturated magmas, consistent with evidence for high temperatures of formation. Crystal sizes in the Coobina chromites show linear trends on crystal-size distribution plots. Such trends are considered to be characteristic of homogeneous populations of growing crystals. There is no evidence of grain-size sorting that might be expected from crystal settling processes, and a mechanism of in situ nucleation and growth is inferred. Chromite seams are interpreted as the result of magma-mixing processes during magma chamber recharge, but with mixing taking place entirely between olivine-saturated magmas, accounting for the lack of pyroxene-bearing cumulates in the sequence, and the higher than usual Cr/Al ratios.

Geology of the Vilama caldera: A new interpretation of a large-scale explosive event in the Central Andean plateau during the Upper Miocene

The Vilama caldera is one of the very large-volume volcanic structures that formed during the ignimbrite flare-up that lasted from 10 Ma to 4 Ma on the central Andean plateau. Formerly, the Vilama structure was interpreted as a 65 × 40 km wide caldera with a two stage evolution. New field correlations, petrographical, geochemical and geochronological data lead to a substantially larger reinterpreted Vilama ignimbrite, whose outcrops cover more than 4000 km 2. New and existing K/Ar and Ar/Ar dating shows that the dacitic Vilama ignimbrite erupted from the Vilama caldera at 8.4–8.5 Ma. The ignimbrite can be divided into extracaldera outflow and intracaldera deposits. The outflow has a mean thickness of ∼ 40 m and is separated into a restricted valley-ponded lower cooling unit, and a laterally extensive low aspect ratio upper cooling unit. The Vilama caldera, which is roughly rectangular (35–40 km × 15–18 km) in shape and has central coordinates of 22°24′ S and 66°57′ W, is considered to have formed in a single-stage collapse event. As the topographic rim is only seen on the western side, the extent of collapse and geometry of the caldera are incompletely known and inferred from indirect data. Possible collapse geometries include a slightly asymmetric single-block subsidence and non-chaotic multiple-block collapse. Estimated erupted volumes range from ∼ 1800 to 1200 km 3, or 1400 to 1000 km 3 in dense rock equivalents. The properties of the Vilama ignimbrite which include a crystal-rich and pumice-poor nature, a high degree of welding and induration and a prodigious volume, suggest that an external drive, rather than volatile overpressures, controlled and maintained the eruption. The best candidate is caldera subsidence triggered by the instability of a magma chamber roof above a batholith-scale magma body. Transtensive/distensive tectonic stresses resulted in the northwest elongation of the magma chamber and/or influenced the roughly rectangular/subelliptical shape of the subsided block. As most of the Vilama ignimbrite is within the caldera, subsidence must have started early in the eruptive history. Immediately before, or concomitantly with the onset of subsidence, the lower extracaldera unit was deposited from flows that formed during collapse of well-developed plumes with efficient convective phases. Once caldera collapse was completely established, the eruption dynamic changed dramatically, and the extracaldera and intracaldera facies erupted. During this phase, unstable and low plumes (boiling-over collapse fountains) preferentially collapsed towards the interior of the subsiding structure, causing the ignimbrite volume to be concentrated in the caldera. After much less than 1 Ma, resurgence, which might be linked to magma chamber recharge, domed the intracaldera facies and caused post-collapse volcanism to be channeled through subsidence and/or resurgence-related faults.

Magma chamber recharge at Vesuvius in the century prior to the eruption of A.D. 79

Evaluating volcanic hazards requires knowledge of the processes that trigger eruptions and the nature and timing of geophysical signals related to these processes. One approach to addressing this need is to link geophysically observable signals (e.g., those related to seismic, aeromagnetic, inflationary, or degassing processes) to pre-eruptive magmatic events deduced (in hindsight) from studies of erupted magmas. Here we present data on sanidine crystals from the A.D. 79 eruption of Vesuvius that show abrupt changes in Ba concentration caused by magma chamber recharge events prior to eruption. These changes have been degraded by diffusion during the time interval between recharge and eruption, and we have determined the length of this time interval by modeling the measured Ba concentration gradients. The results identify three distinct recharge events in the century before the eruption, the most recent occurring ∼20 yr beforehand.

Replenishment episodes and crustal assimilation in the development of an early Tertiary magma chamber, East Greenland: evidence from layered cumulates of the Kælvegletscher ultramafic complex, Kangerlussuaq

From the early Tertiary Kaelvegletscher ultramafic cumulate complex, emplaced into the Archaean basement on the west side of the Kangerlussuaq Fjord, East Greenland, we present geochemical and isotopic data from an outcrop of finely layered dunitic adcumulates. The dunites were deposited in a trough structure, interpreted to be a fossilized feeder channel to the Kaelvegletscher magma chamber. Detailed geochemical sampling of the trough reveals subtle cryptic compositional variations of olivines (Fo = 86.5–89.3; Ni = 2000–2700 ppm) and chromites (Cr# = 66–80) in a stratigraphical profile perpendicular to the layering as well as relatively large simultaneous variations of whole-rock 87Sr/86Sr ratios. The dunites are separable into sequences of normal and reverse cryptic zonations which are interpreted as resulting from fractional crystallisation and magma chamber recharge, respectively. Up to 20% of magma chamber replenishment by high-Mg melts is suggested. Sr-isotope compositions of the cumulates correlate with olivine compositions and suggest mixing of fractionated and unfractionated parental melts assimilating up to 8% local basement. We propose the existence of a vigorous volcanic system at the Kaelvegletscher site in early Tertiary times, where plume-derived magmas incorporated minor amounts of local basement and underwent fractionation and mixing in crustal chambers. Estimates of processed magma volumes during deposition of the trough cumulates are in the range of 100–130 km3.

Pulsed uplift estimated from terrace elevations in the coast of Rome: evidence for a new phase of volcanic activity?

The most recent phase of activity from the Roman volcanoes, since 45 ka, has been interpreted to be the waning stage of the older explosive activity that peaked between 550 and 250 ka. Yet substantial regional uplift from approximately 250 ka suggests that magma chambers beneath the area are growing rather than shrinking. We have estimated the uplift history of the region using radioisotopically and paleomagnetically constrained coastal terrace elevation data that allow us to correlate these terraces with marine oxygen isotopic stages 1–22. Elevations have been corrected for glacio-eustatic sea-level change. Our study shows that over the last million years, regional uplift of coastal terraces occurred in pulses that preceded volcanic activity at roughly 800 ka (in the Monti Sabatini) and 600 ka (in the Alban Hills). The recent pulse of uplift since 250 ka occurred primarily during a period of relative volcanic dormancy that lasted from 250 to 45 ka. We hypothesize that the most recent volcanic activity since 45 ka may reflect the completion of magma chamber recharge, suggesting the start of a new volcanic epoch. It is also possible that the local tectonic stress field changed recently from one dominated by transpression to one dominated by extension, allowing magma to more easily reach the surface via normal faults.

Petrogenesis of an 800 m lava sequence in eastern Uruguay: insights into magma chamber processes beneath the Paraná flood basalt province

Major and trace element along with representative Sr, Nd and Pb isotope data are presented for drill core samples which intersect an 800 m lava pile in eastern Uruguay. The lavas form part of the Paraná flood basalt province, are low-Ti in composition but distinct from the low-Ti Gramado magma type, and have been termed the Treinte Y Trés magma type. The lava pile overlies a large positive gravity anomaly inferred to reflect an east–west trending, mid-crustal mafic intrusive body with a calculated volume of 35,000 km 3. Smooth up-section compositional variations in the basalts are interpreted to record magma evolution within this mid-crustal magma chamber. 87Sr/ 86Sr and 206Pb/ 204Pb increase throughout the sequence yet Mg♯ remains relatively constant in the lower 200 m of the sequence, suggesting a role for magma chamber recharge. Above this the lavas show a regular, up-section decrease in Mg♯ coupled with increasing 87Sr/ 86Sr and 206Pb/ 204Pb and this is interpreted to reflect crystal fractionation combined with crustal contamination. The data provide further evidence that contamination of flood basalt magmas in crustal magma chambers is a common phenomenon and calculations suggest that the amount of crustal addition may be as high as 60–70%. Nevertheless, the effects of this crustal contamination do not appear able to account for the discrepancy between key incompatible trace element ratios and isotope ratios of the lavas and those of any putative mantle plume. In fact, La/Ta decreases with decreasing Mg♯ and increasing 87Sr/ 86Sr indicating that the effects of crustal contamination were actually to reduce La/Ta and implying that the parental magmas had very high La/Ta (∼90). These constraints are clearly inconsistent with an asthenospheric origin for the parental magmas and so, consistent with mass balance calculations, it is inferred that they were derived from the lithospheric mantle.

Compositional and Dynamic Controls on Mafic—Silicic Magma Interactions at Continental Arc Volcanoes: Evidence from Cordón El Guadal, Tatara-San Pedro Complex, Chile

Heterogeneous andesitic and dacitic lavas on Cordón El Guadal bear on the general problem of how magmas of differing compositions and physical properties interact in shallow reservoirs beneath continental arc volcanoes. Some of the lavas contain an exceptionally large proportion (<40%) of undercooled basaltic andesitic magma in various states of disaggregation. Under-cooled mafic magma occurs in the silicic lavas as large (<40 cm) basaltic andesitic magmatic inclusions, as millimeter-sized crystal-clots of Mg-rich olivine phenocrysts plus adhering Carich plagioclase microphenocrysts (An50–70), and as uniformly distributed, isolated phenocrysts and microphenocrysts. Compositions and textures of plagioclase phenocrysts indicate that inclusion-forming magmas are hybrids formed by mixing basaltic and dacitic melts, whereas textural features and compositions of groundmass phases indicate that the andesitic and dacitic lavas are largely mechanical mixtures of dacitic magma and crystallized basaltic andesitic magma. This latter observation is significant because it indicates that mechanical blending of undercooled mafic magma and partially crystallized silicic magma is a possible mechanism for producing the common porphyritic texture of many calc-alkaline volcanic rocks. The style of mafic-silicic magma interaction at Cordon El Guadal was strongly dependent upon the relative proportions of the endmembers. Equally important in the Guadal system, however, was the manner in which the contrasting magmas were juxtaposed. Textural evidence preserved in the plagioclase phenocrysts indicates that the transition from liquid-liquid to solid-liquid mixing was not continuous, but was partitioned into periods of magma chamber recharge and eruption, respectively. Evidently, during periods of recharge, basaltic magmas rapidly entrained small amounts of dacitic magma along the margins of a turbulent injection fountain. Conversely, during periods of eruption, dacitic magma gradually incorporated small parcels of basaltic andesitic magma. Thus, the coupled physical-chemical transition from mixed inclusions to commingled lavas is presumably not coincidental. More likely, it probably provides a partial record of the dynamic processes occurring in shallow magma chambers beneath continental are volcanoes.

Geochemical reversals within the lower 100 m of the Palisades sill, New Jersey

Transects through the lower part of the Palisades sill were made at Fort Lee and Alpine, New Jersey in order to characterize the petrologic signature of previously proposed “reversals” in the normal, tholeiitic differentiation trend. Petrographic and geochemical data include: (1) modal and grain size analyses, (2) bulk rock major and trace element concentrations by DCP-AES, and (3) augite, orthopyroxene, magnetite, and olivine compositions by electron microprobe analysis. Anomalous horizons, defined by increased bulk rock Mg#, Cr, Ni, and Co concentrations and abrupt modal and grain-size changes, occur at 10 m (the well known olivine zone), 27 m, 45 m, and 95m above the basal contact. Thermal models coupled with estimates of the emplacement rate and total magma volume indicate that the olivine zone (OZ) is an early-stage feature, related to the emplacement of initial magma into the Palisades chamber. Stoke's Law calculations indicate that the settling velocity of average-sized olivine crystals in a high-titanium, quartz-normative (HTQ) magma is too slow for significant gravity settling to have occurred prior to the solidification of the basal 20 m of the sill. It is suggested that the OZ resulted from the emplacement of a heterogeneous initial magma from a compositionally stratified, sub-Palisades storage chamber located within the upper crust; however, heterogeneity may have been derived directly from the mantle or during rapid ascent. Geochemical models indicate that the OZ contains accumulated olivine that is not in cotectic (or constant) proportions with the other cumulus phases, suggesting a mechanical sorting process. Magma chamber recharge is proposed to have occurred at the 27 m and 45 m levels, when a slightly more-primitive HTQ magma was injected into the Palisades sill chamber. Zones of elevated Mg# and Cr, 6 to 10 m thick, at these two horizons may indicate the thickness of the hybrid magma formed by the mixing of these two compositions. Geochemical models indicate that the rocks at these levels have accumulated excess orthopyroxene relative to samples from the rest of the sill. Normal faulting in the Fort Lee area at the 95 m level has caused repetition of the stratigraphic section, and hence, the sharp reversal observed at this level.

The open-system geochemical evolution of alkalic cap lavas from Haleakala Crater, Hawaii, USA

Lavas ranging from alkalic basalt to mugearite are exposed in a continuous stratigraphic section in Haleakala Crater on the island of Maui. These lavas define a series of discrete magma batches separated by sharp compositional breaks characterized by upsection increases in incompatible element (e.g., K 2O, Nb, REE, Th, Ba, Rb) contents, Al 2O 3/CaO ratios, and La/Sm ratios and complementary decreases in compatible element (e.g., MgO, Ni, Cr, V, Sc) contents. Within magma batches, there are systematic upsection decreases in incompatible element contents, Al 2O 3/CaO ratios, and La/Sm ratios and systematic increases in compatible element contents. The observed compositional cyclicity can be explained as the result of alternating periods of low and high magma chamber recharge rates. The upsection interbatch compositional breaks represent periods of low or negligible magma chamber recharge, during which eruption is suppressed and the geochemical effects of crystal fractionation dominate those imposed by magma mixing. At the initiation of recharge, the influx of mafic magma into the magma chamber produces an eruption, resulting in partial evacuation of the magma chamber. Following eruption, recharge magma mixes with magma remaining in the magma chamber. Continued recharge and mixing during these eruptive periods cause the compositions of lavas to become more mafic with time. Thus, during a recharge period, the bulk composition of the magma chamber approaches that of the recharge magmas. The most mafic lavas (i.e., the youngest lavas within each magma batch) typically are relatively evolved (Mg# < 48, MgO < 7 wt%) and, therefore, cannot represent primary magmas. Evidence for polybaric fractionation suggests that these lavas represent recharge magmas that have undergone crystal fractionation in a deeper-level reservoir prior to injection into the shallow-level Haleakala magma chamber. The observed fine-scale geochemical cyclicity is superimposed over the larger-scale, systematic changes in isotope ratios and highly incompatible trace element ratios that are attributed to time-dependent changes in the proportions of distinct mantle components in the source of these lavas. These contrasting scales of temporal geochemical variability illustrate how shallow-level magma chamber processes have imposed short time period (≈ 2000 years) compositional controls on magmas whose compositions were also changing over relatively long time periods (≈ 800,000 years) due to source-related effects. The decrease in magma supply rates during the alkalic cap phase of Hawaiian volcanism probably results in increased magma chamber residence times, thus creating the greater magmatic diversity observed. The specifics of the petrologic and geochemical evolution of individual Hawaiian volcanoes during this period differ at least in part because of differences in magma supply rates.

Interpretation of open system petrogenetic processes: Phase equilibria constraints on magma evolution

We have used a computer model (TRACES) to simulate low pressure differentiation of natural basaltic magmas in an attempt to investigate the chemical dynamics of open system magmatic processes. Our results, in the form of simulated liquid lines of descent and the calculated equilibrium mineralogy, were determined for perfect fractional crystallization; fractionation paired with recharge and eruption (PRF); fractionation paired with assimilation (AFC); and fractionation paired with recharge, eruption, and assimilation (FEAR). These simulations were calculated in an attempt to assess the effects of combinations of petrogenetic processes on major and trace element evolution of natural systems and to test techniques that have been used to decipher the relative roles of these processes. If the results of PRF calculations are interpreted in terms of a mass balance based fractionation model (e.g., Bryan et al., 1969), it is possible to generate low residuals even if one assumes that fractional crystallization was the only active process. In effect, the chemical consequences of recharge are invisible to mass balance models. Pearce element ratio analyses, however, can effectively discern the effects of PRF versus simple fractionation. The fractionating mineral proportions, and therefore, bulk distribution coefficients (D) of a differentiating system are dependent on the recharge or assimilation rate. Comparison of the results of simulations assuming constant D with the results calculated by TRACES show that the steady state liquid concentrations of some elements can differ by a factor of 2 to 5. If the PRF simulation is periodic, with episodes of mixing separated by intervals of fractionation, parallel liquidus mineral control lines are produced. Most of these control lines do not project back to the parental composition. This must be an important consideration when attempting to calculate a potential parental magma for any natural suite where magma chamber recharge has occurred. Most basaltic magmas cannot evolve to high silica compositions without magnetite fractionation. Small amounts of rhyolite assimilation (assimilation/fractionation < 0.1), however, can drive evolving basalts to more silica rich compositions. If mass balance models are used to interpret these synthetic AFC data, low residuals are obtained if magnetite is added to the crystallizing assemblage. This approach works even for cases where magnetite was not a fractionating phase. Thus, the mass balance results are mathematically correct, but are geologically irrelevant.

TRACE FOR: A program for the calculation of combined major and trace-element liquid lines of descent for natural magmatic systems

A two-lattice model was used to calculate melt component activities, allowing for the calculation of compositionally independent partition coefficients. The elimination of compositional dependence leaves the partition coefficients of isobaric experiments dependent upon temperature alone. The temperature dependence can be described for many trace-element partition coefficients ( D) by a linear expression relating In D to reciprocal temperature. The combination of these trace-element expressions with a model of major-element phase equilibria has resulted in a model designed to calculate the differentiation paths of mafic and intermediate compositions as they undergo some combination of fractional crystallization, assimilation, magma chamber recharge, and tapping of the magma chamber. This program, TRACE FOR, written in FORTRAN is available on diskette for IBM PC compatible systems.

A model for the simulation of combined major and trace element liquid lines of descent

Variations of major and trace element partition coefficients as functions of temperature and composition have severely limited the range of composition for quantitative modeling of the differentiation of natural magmatic systems. Application of a two-lattice melt model greatly reduces compositional dependence for trace elements. This allows the temperature dependence of isobaric trace element partitioning to be evaluated independently, thus making it possible to model an expanded range of conditions and compositions. The first goal of this investigation was to construct a model to simulate major and trace element systematics of natural differentiation processes, by combining trace element partitioning with a major element phase equilibrium model ( Nielsen, 1985b). The second goal was the application of this model to the simulation of magma chamber recharge, assimilation, eruption and fractional crystallization. These simulations indicate that the proportions of minerals fractionating from an open system are not necessarily the same as those in a closed system. Recharge or assimilation will drive a magma composition off the cotectic along which it is evolving. However, fractionation will drive the liquid back towards the cotectic. In a recharged magma chamber, as the recharge flux increases, the fractionating mineral proportions will be increasingly biased towards those phases which appear first from the primitive magma. The bulk partition coefficient for an open system can be considerably different (5–10× or more), for some elements, compared to a closed system. Paired assimilation and fractional crystallization are often cited as the mechanisms responsible for the origin of high Al basalts and andesites. However, the addition of an Al-rich assimilant into a fractionating basaltic magma will increase the proportion of plagioclase to the extent that Al in the derived liquids will be effectively buffered, or even decrease. The assimilation of forsteritic olivine by reaction ( Keleman and Ghiorso, 1986), increases the proportions of pyroxene and olivine, thus reducing the Fe-enrichment and Al-depletion characteristic of low pressure basalt fractionation.

EQUIL: a program for the modeling of low-pressure differentiation processes in natural mafic magma bodies

The use of a two-lattice model for the activity of silicate melt components permits temperature dependent, compositionally independent distribution coefficients to be derived. The compositional independence of these functions allows a model to be constructed of phase equilibria applicable to a wide range of natural mafic systems. Using the phase equilibria model as a base, the program EQUIL was constructed. Given an initial primitive parental mafic composition, EQUIL enables one to model most low-pressure differentiation processes involving fractionation. These include magma chamber recharge, fractional or equilibria crystallization, assimilation and tapping of the magma chamber. Lines of descent can be calculated for a magma body undergoing a single differentiation process, or several processes together (i.e. paired assimilation and fractional crystallization).